An anti-lock control system is among the most safety-relevant devices in a motor vehicle. Since the lives and health of a driver and passengers are directly dependent upon flawless functioning of such a system, it is necessary to carefully monitor operation of the entire system and individual components thereof. In this regard, it is particularly important to monitor magnetic valves, which directly engage a hydraulic brake system, and the circuit amplifiers which control the magnetic valves.
This invention relates to a circuit for controlling and monitoring inductive loads, particularly magnetic valves in an anti-lock control system, comprising at least one circuit amplifier for coupling at least one inductive load to a voltage source and a microcomputer controlling the at least one circuit amplifier and monitoring a voltage status at an interface between the circuit amplifier and the inductive load.
Such a circuit is known, for example, from German patent document DE-OS 39 28 651. There, a microcomputer controls a load by means of an output amplifier. A control lead transmits a voltage signal to the microcomputer from an interface between the circuit amplifier and the load. In addition, a resistor arrangement and a switching device are provided which, also, under control of the microcomputer, read voltages at various points in the load circuit and transmit this information to the microcomputer via an additional signal lead. Further, it is provided for the microcomputer to control the output amplifier one time, or on a particularly regular basis, that is, even during an anti-lock control process, for a period, or periods, of time short enough not to activate the magnetic valve. Meanwhile, the microcomputer reads the voltages at various points by means of a switching circuit, or device. Thereby, the microcomputer can detect short circuits and/or lead interruptions within the load circuit made up of the output amplifier and the magnetic valve.
Such an arrangement is quite cost-intensive with respect to hardware and software. That is, in addition to the output amplifier, a resistor arrangement and a switching circuit, or device, are required. This necessitates at least two additional terminals of the microcomputer for each magnetic valve monitored, because there must be a control lead from the microcomputer to the switching device and a lead returning detected voltage signals to the microcomputer.
In addition, continuous monitoring requires additional software, particularly for monitoring the voltage also during an anti-lock control process. In this regard, the microcomputer must drive the output amplifier for a short period of time, practically inbetween other demanding control processes, without operating the magnetic valve. Also, the microcomputer must control the switching device and detect various voltages without negatively influencing the anti-lock control process, which is complicated enough in itself.
It is a disadvantage of this arrangement that both the controlling and the monitoring of the circuit amplifier are performed only by the microcomputer. If the output amplifier shows a high power loss, for example, due to an incompletely-controlled final-stage transistor or due to a closing of, or a short in, a winding of the magnetic valve coil, the speed of fault detection and a resulting termination of the control of the output amplifier is particularly dependent upon a program performance time of the microcomputer. This time is in the range of several milli-seconds and is generally not short enough to prevent destruction of the transistor due to increased power loss. Thus, one of the mentioned malfunctions leads almost inevitably to the destruction of the output amplifier.
It is an object of this invention to provide a circuit for controlling and monitoring inductive loads, particularly magnetic valves in an anti-lock control system, which is constructed in an uncomplicated and cost-effective manner and provides higher operation safety than circuits of the prior art and allows controlling and monitoring of a load circuit by means of a microcomputer in a less complicated, and less cost-intensive, manner.